Energy & Fuels 2008, 22, 1055–1058
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Discussion on “Characteristics of Fly Ashes from Full-Scale Coal-Fired Power Plants and Their Relationship to Mercury Adsorption” by Lu et al. James C. Hower,*,† Bruno Valentim,‡ Irena J. Kostova,§ and Kevin R. Henke† UniVersity of Kentucky Center for Applied Energy Research, 2540 Research Park DriVe, Lexington, Kentucky 40511, Department of Geology, UniVersity of Porto, rua do Campo Alegre, 687, 4169-007 Porto, Portugal, and Sofia UniVersity “St. Kliment Ohridski”, 15, Tzar OsVoboditel BlVd., 1000 Sofia, Bulgaria ReceiVed NoVember 14, 2007. ReVised Manuscript ReceiVed December 26, 2007
Mercury capture by coal-combustion fly ash is a function of the amount of Hg in the feed coal, the amount of carbon in the fly ash, the type of carbon in the fly ash (including variables introduced by the rank of the feed coal), and the flue gas temperature at the point of ash collection. In their discussion of fly ash and Hg adsorption, Lu et al. [Energy Fuels 2007, 21, 2112-2120] had some fundamental flaws in their techniques, which, in turn, impact the validity of analyzed parameters. First, they used mechanical sieving to segregate fly ash size fractions. Mechanical sieving does not produce representative size fractions, particularly for the finest sizes. If the study samples were not obtained correctly, the subsequent analyses of fly ash carbon and Hg cannot accurately represent the size fractions. In the analysis of carbon forms, it is not possible to accurately determine the forms with scanning electron microscopy. The complexity of the whole particles is overlooked when just examining the outer particle surface. Examination of elements such as Hg, present in very trace quantities in most fly ashes, requires careful attention to the analytical techniques.
We welcome studies of power plants and the complex interrelationship between fly ash carbon, the flue gas temperature at the point of fly ash collection, and the amount of Hg captured by the fly ash. Studies of other power plants should enhance our understanding of the relationships observed in the plants in our studies. The study by Lu et al.,1 however, while addressing some important issues, is flawed in many respects. What is known about the inter-relationship between fly ash carbon and Hg capture? It has been established in a number of previous studies that the behavior of Hg in flue gas and its capture by fly ash differs from other volatile trace elements.2–13 * Corresponding author. Tel.: 1 + 859-257-0261. E-mail:
[email protected]. † University of Kentucky Center for Applied Energy Research. ‡ University of Porto. § Sofia University “St. Kliment Ohridski”. (1) Lu, Y.; Rostam-Abadi, M.; Chang, R.; Richardson, C.; Paradis, J. Characteristics of fly ashes from full-scale coal-fired power plants and their relationship to mercury adsorption. Energy Fuels 2007, 21, 2112–2120. (2) Senior, C. L.; Bool, L. E.; Morency, J. R. Laboratory study of trace element vaporization from combustion of pulverized coal. Fuel Process. Technol. 2000, 65–66, 109–124. (3) Senior, C. L.; Helble, J. J.; Sarofim, A. F. Emissions of mercury, trace elements, and fine particles from stationary combustion sources. Fuel Process. Technol. 2000, 65–66, 263–288. (4) Senior, C. L.; Sarofim, A. F.; Zeng, T.; Helble, J. J.; Mamani-Paco, R. Gas-phase transformations of mercury in coal-fired power plants. Fuel Process. Technol. 2000, 65–66, 197–213. (5) Lindau, L. Mercury sorption to coal fly ash. Staub-Reinhaltung der Luft 1983, 43, 166–167. (6) Sen, A. K.; De, A. K. Adsorption of mercury (II) by coal fly ash. Water Res. 1987, 21, 885–888. (7) Hassett, D. J.; Eylands, K. E. Mercury capture on coal combustion fly ash. Fuel 1999, 78, 243–248. (8) Gibb, W. H.; Clarke, F.; Mehta, A. K. The fate of coal mercury during combustion. Fuel Process. Technol. 2000, 65–66, 365–377. (9) Serre, S. D.; Silcox, G. D. Adsorption of elemental mercury on the residual carbon in coal fly ash. Ind. Eng. Chem. Res. 2000, 39, 1723–1730.
Specifically, Hg capture by fly ash is a function of the amount of carbon in the fly ash, with Hg capture generally increasing with a rise in C at the same flue gas temperature.14–20 A (10) Meij, R.; Vredenbregt, L. H. J.; te Winkel, H. The fate and behavior of mercury in coal-fired power plants. J. Air Waste Manage. Assoc. 2002, 52, 912–917. (11) Sloss, L. L. Mercury-emissions and controls. IEA Coal Res. 2002, CCC/58, 43. (12) Tan, Y.; Mortazivi, R.; Dureau, B.; Douglas, M. A. An investigation of mercury distribution and speciation during coal combustion. Fuel 2004, 83, 2229–2236. (13) Li, J.; Gao, X.; Goekner, B.; Kollakowsky, D.; Ramme, B. A pilot study of mercury liberation and capture from coal-fired power plant fly ash. J. Air Waste Manage. Assoc. 2005, 55, 258–264. (14) Hower, J. C.; Trimble, A. S.; Eble, C. F.; Palmer, C.; Kolker, A. Characterization of Fly Ash from Low-sulfur and High-sulfur Coal Sources: Partitioning of carbon and trace elements with particle size. Energy Sources 1999, 21, 511–525. (15) Hower, J. C.; Robl, T. L.; Anderson, C.; Thomas, G. A.; Sakulpitakphon, T.; Mardon, S. M.; Clark, W. L. Characteristics of CCP’s from Kentucky power plants, with emphasis on Mercury content. Fuel 2005, 84, 1338–1350. (16) Mardon, S. M.; Hower, J. C. Impact of coal properties on coal combustion by-product quality: Examples from a Kentucky power plant. Int. J. Coal Geology 2004, 59, 153–169. (17) Mastalerz, M.; Drobniak, A.; Lis, G.; Hower, J. C.; Mardon, S. M. Chemical properties and petrographic composition of coal and fly ash: Examples from Indiana mines and power plants. Int. J. Coal Geology 2004, 59, 171–192. (18) Sakulpitakphon, T.; Hower, J. C.; Trimble, A. S.; Schram, W. H.; Thomas, G. A. Mercury capture by fly ash: Study of the combustion of a high-mercury coal at a utility boiler. Energy Fuels 2000, 14, 727–733. (19) Hower, J. C.; Finkelman, R. B.; Rathbone, R. F.; Goodman, J. Intraand Interunit Variation in Fly Ash Petrography and Mercury Adsorption: Examples from a Western Kentucky Power Station. Energy Fuels 2000, 14, 212–216. (20) Sakulpitakphon, T.; Hower, J. C.; Schram, W. H.; Ward, C. R. Tracking mercury from the mine to the power plant: geochemistry of the Manchester coal bed, Clay County, Kentucky. Int. J. Coal Geology 2004, 57, 127–141.
10.1021/ef700683q CCC: $40.75 2008 American Chemical Society Published on Web 02/16/2008
1056 Energy & Fuels, Vol. 22, No. 2, 2008
Hower et al.
Table 1. Comparison of the Weight Percentages in Screen Fractions of an Economizer Fly Ash with the Volume Percentage of the Same Ash Analyzed by Laser Granulometrya size (mm)
wet screening (wt %)
dry screening (wt %)
laser granulometry (vol %)
150
25 18 24 18 8
10 13 25 24 29
16.47 13.20 18.11 28.12 21.84
a
Data are from the work of Valentim.35
relationship between fly ash carbon type, BET surface area, and Hg capture has also been noted, 21,22 with further complications with carbons derived from subbituminous coals23–25 and anthracites,26,27 in contrast to the high volatile bituminous coalderived carbons used by Hower and Maroto-Valer in their investigations.21,22 The proper presentation and discussion of such relationships is dependent on the proper collection and handling of samples. Lu et al.1 did learn something of the boiler types and conditions and of the flue gas temperatures of the systems in question, but the handling of the samples offset some of the advantages of their thoroughness. In particular, the use of mechanical sieving (p 2113) for the splitting of fly ash is suspect. Although wet screening is inappropriate for high-alkali class C fly ashes that may contain water-soluble (e.g., Na2SO4) or reactive (e.g., CaO) compounds, the method has been successfully used with class F ashes. In comparison, mechanical sieving is known to under-represent the finest class F fly ash sizes when compared to wet screening. Valentim35 (Table 1) found that dry screening of an economizer fly ash indicated that only 9.6% of the ash reported to the
